Didot point

Point (typography)

In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The traditional printer's point, from the era of hot metal typesetting and presswork, varied between 0.18 and 0.4 mm depending on various definitions of the foot.

Today, the traditional point has been supplanted by the desktop publishing point (also called the PostScript point), which has been rounded to an even 72 points to the inch (1 point = mm = 352.7 µm). In either system, there are 12 points to the pica.

French printer’s points

See French units of measurement for the definitions of the units used in this section.

A French law of 1799 defined the metre to be exactly 443.296 French lines—or 3 French feet, 0 French inches and 11.296 French lines. Since the metre is now the standard unit, this implicitly defines the Pied du Roi, or French Royal foot, as exactly metres, or about 324.839 385 mm. This value is used in the conversions below.

Truchet

The modern typographic point was invented in France by the clergyman Sébastien Truchet (1657–1729). The size he chose was such that 1728 of these made one Pied du Roi— using the 1799 definition, mm (or about 187.985 755 2 µm).

Fournier

Pierre Simon Fournier (1712–1768) used a typographic point of about French Royal inches ≈ 345 µm. Fournier’s point did not achieve lasting popularity.

Didot

François-Ambroise Didot (1730–1801) returned to Truchet’s idea, but chose a size twice as large. Thus 864 of his points made one Pied du Roi—that is, .

This value—somewhat odd due to the divisor, which has the prime factorization 3 × 7 × 1979—was not very flexible for use by typesetters and printers. Though the general size of the Didot point continued to be preferred to that of Truchet, several other printers each chose his or her own value for the point. These are compared below:

376.065 µm (0.0249% larger than Didot's point)—the traditional value in European printers' offices
376.000 µm (0.0076% larger)—used by H. Berthold AG (1831–1904) and many others
375.940 µm (0.0084% smaller)—Jan Tschichold (1902–1974), who used 266 points in 100 mm
375.000 µm (0.2584% smaller)—proposed in 1975, but never adopted

Note that the Imprimerie nationale adopted a point of 400 µm exactly, and continues to use this measurement today.

The Didot point has been replaced by the DTP point in France and throughout the world.

Traditional American point system

By the (Kasson) Metric Act of 1866 (Public Law 39-183), the US (survey) foot is m. This is 0.0002% more than 304.8 mm, which is the length of the Anglo-Saxon compromise foot of 1959, used below. A typographic foot contains 72 picas or 864 points.

Nelson C. Hawks, in 1879, used a printer’s foot of an Anglo-Saxon foot decreased by 0.375%. Therefore, the traditional ratio (which reduces to ) places Hawks’ point at 0.013 837 inch, or about 351.46 µm.
A second definition was proposed whereby there were exactly 996 printer’s points (= 83 picas) in 350 mm, which made the printer’s point about 0.013 848 867 inch ≈ 351.405 622 µm.
Finally, Lawrence Johnson stated in a third definition of printer’s foot that it should be (99.6%) English foot. This means that the Johnson’s typographical point was 0.01383 inch, and was then converted by the 1959 value to 351.36 µm.

In 1886, the Fifteenth Meeting of the Type Founders Association of the United States approved the so-called Johnson pica be adopted as the official standard. This makes the traditional American printer’s foot measure , or 303.5808 mm exactly, giving a point size of approximately of an inch, or 351.5 µm.

This is the size of the point in the TeX computer typesetting system by Donald Knuth, which predates Postscript slightly. Thus the latter unit is commonly called the TeX point.

Like the French Didot point, the traditional American printer’s point was replaced in the 1980s by the current computer-based DTP point system.

Current DTP point system

The desk-top publishing point (DTP point) is defined as 1/72 of the anglo-Saxon compromise inch of 1959 (25.4 mm), it is approximately 0.0139 inch or 352.8 µm. Twelve points make up a pica, and six picas make an inch.

The point is the standard unit for measuring font size and leading and other minute items on a printed page. This system was notably promoted by John Warnock and Charles Geschke, the inventors of Adobe PostScript, and therefore it is sometimes also called PostScript point.

In metal type, the point size of the font described the size (height) of the metal body on which the typeface's characters were cast. In digital type, the body is now an imaginary design space, but is used as the basis from which the type is scaled (see em).

A measurement in picas is usually represented by placing a lower case p after the number, such as "10p" means "10 picas". Points are represented by placing the number of points after the p, such as 0p5 for "5 points," 6p2 for "6 picas and 2 points", or 1p1 for "13 points" which is converted to a mixed fraction of 1 pica and 1 point. (An alternate nomenclature is described in the pica article.)